The present invention relates to soldering techniques and, more particularly, to a solder method that enhances solder interconnects by eliminating solder joint failures caused by micro-cracking at or near the solder intermetallic interface.
The fatigue life of solder interconnects is often poor, because cracks develop near an intermetallic layer. The damaging process is due to the build-up of inelastic deformation (creep) that leads to cavity nucleation, growth, and coalescence along grain boundaries. The increasing damage tends to produce micro-cracks at the boundaries. These boundary micro-cracks are disposed roughly normal to the direction of maximum tensile stress.
The factors that influence the aforementioned damage include: (a) the shape of the joint, which influences the stress concentration at the free joint boundaries; (b) the build-up of intermetallics, which are known to locally increase stress in solder at and above the intermetallic layer; and (c) the local coefficient of thermal expansion (CTE) mismatch between the pad and the solder.
It is also observed that dissolved copper, gold, or other metallic pad coating materials locally contaminate solder. The contaminants increase the solder brittleness, making the solder susceptible to micro-cracking, when compared with bulk behavior.
The present invention seeks to increase the fatigue life of the solder joint, by limiting the damage caused by micro-cracking in the solder joint. This objective is achieved by redistributing the stresses in solder, thus constraining the cracks. Such containment can be accomplished by creating obstacles along the crack path, redirecting the crack away from the intermetallic layer, or by increasing the path length along which the crack is to propagate. The solder layer can be designed to include a serpentine, interrupted, or interdigitated boundary. The method can be applied to ball grid arrays, column grid arrays, surface mount technology (SMT) joints, etc.
In U.S. Pat. No. 5,242,569, issued to Kang et al, on Sept. 7, 1993, for THERMOCOMPRESSION BONDING IN INTEGRATED CIRCUIT PACKAGING, a thermocompression bonding method is described that allows bonding to be achieved at lower temperatures. The process produces a soft, deformable layer of metal that is free of dendritic protrusions.
In U.S. Pat. No. 5,172,473, issued to Burns et al, on Dec. 22, 1992, for METHOD OF MAKING CONE ELECTRICAL CONTACT, a method of achieving improved electrical contact is illustrated. Contact is improved by generating cone-shaped projections upon a mating surface. The cones enhance ohmic contact by intermeshing and wiping.
In U.S. Pat. No. 5,118,299, issued to Burns et al, on Jun. 2, 1992, for CONE ELECTRICAL CONTACT, an electrical interconnection is shown featuring two detachable surfaces having intermeshing cone projections. The cones
In U.S. Pat. No. 3,881,799, issued on May 6, 1975, to Elliott et al, for RESILIENT MULTI-MICRO POINT METALLIC JUNCTION, a dynamic interfacing contact device is disclosed. The device provides multiple points of contact between opposing parallel surfaces of a pair of conductors. The points of contact are provided by a number of spaced-apart, metal protrusions.
In U.S. Pat. No. 4,751,563, issued to the common assignee, a microminiaturized electrical interconnection device is described. Electrical connection on a first pad is tangentially raised at about sixty degrees and brought into intimate contact with a second metallic layer.
In accordance with the present invention, there is provided a method and article of fabrication, featuring a solder layer that comprises a serpentine, interrupted, or interdigitated boundary. The non-planar design of the boundary layer increases the fatigue life of the solder joint, by limiting the damage caused by micro-cracking. This irregularity of the solder boundary constrains the propagation of cracks by creating obstacles along the crack path, redirecting the crack away from the intermetallic layer, or by increasing the path length along which the crack propagates.
It is an object of this invention to provide a method and article of fabrication that improves the fatigue life of solder joints.
It is another object of the invention to produce a solder joint that constrains cracking along the intermetallic boundary.